Currently, a conventional 56K analog modem for making connections over an analog plain old telephone system (POTS) line may use the V.90 standard, which refers to the International Telecommunication Union specification (ITU) V.90 series 1, announced on Feb. 6, 1998. A V.90 modem uses pulse code modulation (PCM) in the downstream direction (receiving direction) to achieve up to 56K bits-per-second data speed. FIG. 1 illustrates a prior art connection between analog modems 120 and a digital modem 100. The modems 100,120 are connected through an intermediary telephone switch 110, such as a standard class 5 switch at the telephone system office. That is, the analog modem at the user end dials a number and connects to the class 5 switch, which is then connected to the digital modem at the Internet Service Provider (ISP) end via a digital serial data link line, such as a T1 connection.
Telephone systems employing a universal digital loop carrier (UDLC) system are unable to make V.90 (PCM downstream) modem connections. Very often in UDLC connections, a central office line unit (COLU) and a remote terminal (RT) are added in the telephone connection path (see, for example. FIG. 2). Because there are two analog signal paths along the two analog POTS line connections (between the analog modems and the remote terminal, and between the COLU and the telephone switch), V.90 connections are not possible without further modification of the UDLC system because of the line noise and other factors that may distort the signal. In order to overcome this limitation, manufacturers of UDLC systems have added in their UDLC equipment a downstream equalizer, a downstream canceller, and an upstream canceller to improve the chances of making proper V.90 PCM connections. V.90 series 1 modems have a PCM modem only in the downstream direction, and use a slower analog modem in the upstream direction. However, even if analog modems are later developed utilizing a protocol where PCM signals are also transmitted in the upstream direction, current UDLC systems are not adapted to handle upstream PCM signals.
According to an embodiment of the present invention, a central office line unit (COLU) for a pulse code modulation (PCM) upstream and downstream system in a universal digital loop carrier (UDLC) includes a hybrid circuit for transmitting/receiving an analog signal to/from a switch. An analog-to-digital converter is provided for receiving the analog signal from the hybrid circuit and converting the analog signal to a converted digital signal. A digital transmission circuit, such as a high-speed digital subscriber line (HDSL) circuit transmits/receives a digital signal to/from a remote terminal. A quantizer is provided for evaluating and processing the converted digital signal so that only valid states of the converted digital signal are transmitted to the digital transmission circuit for transmission to the remote terminal. A receiver is used for receiving the digital signal from the digital transmission circuit and for evaluating and processing the digital signal so that only valid states of the digital signal are outputted from the receiver. A transmitter receives the digital signal from the receiver and equalizes the digital signal prior to transmitting the digital signal to a digital-to-analog converter. The digital-to-analog converter converts the digital signal to a converted analog signal and transmits the converted analog signal to the hybrid circuit, which transmits the converted analog signal to the switch.
According to another embodiment of the present invention, a central office line unit (COLU) for a pulse code modulation (PCM) upstream and downstream system in a universal digital loop carrier (UDLC) includes a hybrid circuit for transmitting/receiving an analog signal to/from a switch. An analog-to-digital converter is provided for receiving the analog signal from the hybrid circuit and converting the analog signal to a converted digital signal. A digital transmission circuit transmits/receives a digital signal to/from a remote terminal. A quantizer is provided for evaluating and processing the converted digital signal so that only valid states of the converted digital signal are transmitted to the digital transmission circuit for transmission to the remote terminal.
According to another embodiment of the present invention, a central office line unit (COLU) for a pulse code modulation (PCM) upstream and downstream system in a universal digital loop carrier (UDLC) includes a hybrid circuit for transmitting/receiving an analog signal to/from a switch. An digital transmission circuit transmits/receives a digital signal to/from a remote terminal. A receiver is provided for receiving the digital signal from the digital transmission circuit and for evaluating and processing the digital signal so that only valid states of the digital signal are outputted from the receiver. A transmitter is provided for receiving the digital signal from the receiver and for equalizing the digital signal. A digital-to-analog converter converts the equalized digital signal to a converted analog signal. The converted analog signal is sent to the hybrid circuit, which transmits the converted analog signal to the switch.
According to another embodiment of the present invention, a remote terminal for a pulse code modulation (PCM) upstream and downstream system in a universal digital loop carrier (UDLC) includes a receiver for receiving a digital signal converted from an analog signal of an analog modem, and for evaluating and processing the digital signal so that only valid states of the digital signal are outputted from the receiver. A transmitter is provided for receiving the digital signal from the receiver, and for equalizing the digital signal prior to transmitting the digital signal to a digital-to-analog converter in a central office line unit (COLU).
According to another embodiment of the present invention, a pulse code modulation (PCM) upstream and downstream system in a universal digital loop carrier (UDLC) includes at least one analog modem connected to an analog plain old telephone system (POTS) line. A remote terminal is connected to the analog modem via the analog POTS line and further connected to a high-speed digital subscriber line. A central office line unit is provided, which includes a hybrid circuit for transmitting/receiving an analog signal to/from a switch. The switch is connected to the hybrid circuit via a second analog plain old telephone system (POTS) line. An analog-to-digital converter is provided for receiving the analog signal from the hybrid circuit and converting the analog signal to a converted digital signal. A digital transmission circuit transmits/receives a digital signal to/from the remote terminal. A quantizer is provided for evaluating and processing the converted digital signal so that only valid states of the converted digital signal are transmitted to the digital transmission circuit for transmission to the remote terminal. A receiver receives the digital signal from the digital transmission circuit, and evaluates and processes the digital signal so that only valid states of the digital signal are outputted from the receiver. A transmitter is provided for receiving the digital signal from the receiver, and for equalizing the digital signal. A digital-to-analog converter converts the digital signal to a converted analog signal and transmits the converted analog signal to the hybrid circuit, which transmits the converted analog signal to the switch via the second analog POTS line. A digital modem is connected to the switch via a digital serial data link line.
According to another embodiment of the present invention, a method of transmitting a downstream pulse code modulation (PCM) signal in a PCM upstream and downstream system in a universal digital loop carrier (UDLC) includes receiving a downstream digital signal from an analog-to-digital converter. The downstream digital signal is evaluated for an invalid state not designated by the PCM upstream and downstream system. Then, the downstream digital signal is processed so that only valid states designated by the PCM upstream and downstream system are transmitted downstream.
According to yet another embodiment of the present invention, a method of transmitting an upstream pulse code modulation (PCM) signal in a PCM upstream and downstream system in a universal digital loop carrier (UDLC) includes receiving an upstream digital signal from an analog-to-digital converter. The upstream digital signal is evaluated for invalid states not designated by the PCM upstream and downstream system. The upstream digital signal is processed so that only valid states designated by the PCM upstream and downstream system are transmitted. The upstream digital signal is equalized and then transmitted upstream.